Such transistors are of the type which comprises a substrate of a semiconductor material having a low concentration of a dopant of a first type, a first layer, or bottom well layer, extending over the substrate and having a low concentration of a dopant of a second type, an intermediate region overlying said first, bottom well layer and being higher in concentration of the first-type dopant, and an epitaxial layer doped with the second-type dopant and covering the substrate and said region to define a transistor surface. Respective emitter, base, and collector regions of said transistor which are separated structurally are formed in said epitaxial layer, and opposedly located isolation zones are provided whose depth extends to the substrate and which are effective to border said regions laterally in the epitaxial layer, the collector region extending in depth to penetrate said intermediate region.
As is well known, vertical isolated-collector pnp transistors are widely employed in a very large number of circuit arrangements specifically intended for operation on a relatively high supply voltage.
However, the performance of such transistors and their characteristically good frequency response are affected by the presence, within their physical structure, of active parasitic junction components such as transistors or silicon-controlled rectifiers (SCR's).
In relation to the structure of the vertical isolated-collector pnp transistor referred to above, it may be appreciated that a parasitic component exists therein which consists of a transistor of the pnp type having its base open into the epitaxial layer, its emitter formed by the collector region of the vertical pnp, and its collector formed in an isolation zone.
The effect of that component could be one of delivering undesirable leakage currents to the substrate.
A first prior approach directed to obviating the presence of that parasitic transistor has been that of shorting out its base-emitter junction by biasing the epitaxial layer to the same potential as the collector of the vertical pnp transistor.
To this aim, there has been formed between the collector region and one of the isolation zones a small island doped with the same dopant type as the epitaxial layer, but having a higher dopant concentration. That island is effective to create an area of contact with the epitaxial layer to permit interconnection of that epitaxial layer and the collector terminating pin, externally of the transistor.
However, there is present in the vertical isolated-collector pnp transistor a second parasitic semiconductor device of the pnpn type comprising four superimposed emitter layers, a top well epitaxial layer, intermediate region, and bottom well layer. That device is a silicon-controlled rectifier of a parasitic type whose effect may well be that of degrading the transistor characteristics.
It is in the very attempt at suppressing the effect of the first parasitic transistor by shorting out the base-emitter junction in accordance with the prior technique that said silicon-controlled rectifier is turned on where the product of the collector current by the internal resistance of the collector region of the vertical pnp transistor equals the voltage drop between the base and the emitter of the parasitic transistor.
Furthermore, since the value of that resistance is quite a varying one even between spots on the semiconductor, the problem is encountered that the value of the collector current which triggers on the parasitic rectifier is undetermined.